Zinc deficiency, and not lead poisoning from canned food, may have caused the death of the 129 crew members of the Franklin expedition, says analysis of 170-year old nail fragments from one of the sailors.

Led by Captain Sir John Franklin, the British voyage departed England bound for the Arctic in 1845 aboard two ships, HMS Erebus and HMS Terror.

The mission was to find the fabled Northwest Passage - the shortcut between Britain and Asia. But all crew members vanished in the ice-choked Arctic. Stories handed down by the Inuit people tell of the seamen's desperate final months, with tales of some even resorting to cannibalism.

The expedition's loss at sea was one of the most celebrated mysteries of the Victorian era and a puzzle that continues to baffle naval historians.

The analyzed big toenail and thumbnail belonged to able seaman John Hartnell, one of the crew members of the doomed Arctic expedition. The nail fragments were analyzed with lasers and the University of Saskatchewan's synchrotron particle accelerator.

Hartnell died during the expedition's first overwintering on Beechey Island in 1845–46, and was buried there. The study turned his nails into a time machine, revealing changes in his body week-by-week.

As nails grow at a rate of about three millimeters a month, they store biochemical information, revealing how metal concentrations change in the body over time.

"We estimated that the thumbnail represented the time period June 22, 1845 to January 4, 1846, and the big toenail represented the time period May 23, 1845 to January 4, 1846," the researchers wrote in the Journal of Archaeological Science: Reports.

The study found that Hartnell was chronically zinc-deficient. Zinc plays an integral role in vitamin A metabolism, and deficiencies in zinc and vitamin A can result in compromised immune functions. This made the sailor more vulnerable to tuberculosis and pneumonia, diseases that are believed to have killed him.

"The process of starvation from tuberculosis in Hartnell resulted in the exponential release of previously stored lead from his bones into the blood," first author Jennie Christensen, an associate of Stantec Consulting in Sidney, Canada, and CEO and Founder of TrichAnalytics in North Saanich, Canada, said.

She noted that lead concentrations were only high and increasing at the end of his life when he was already likely near death.

"This explains why previous researchers discovered high lead concentrations in soft tissue; however, they erroneously concluded it was due to recent exposure," Christensen said.

The researchers believe that other factors, including poor diet, played important roles in the crew's health.

The nail fragments showed that Hartnell did not eat meat or seafood in his final days, a unusual fact since both ships were well-stocked with tinned meat. Most likely, the canned food was spoiled and the sailors were unable to get seafood.

With the exception of Hartnell and the other two sailors who succumbed to tuberculosis and pneumonia on Beechey Island, causes of death are unknown for the other crew members. No other nail samples are available for further investigation.

"We suggest it is likely that other crewmen on the expedition were also malnourished, zinc deficient, immuno-suppressed, and exposed to the bacterium responsible for tuberculosis, and these factors likely played a role in their untimely deaths," the researchers concluded.

They noted that malnourishment and zinc deficiency can have behavioral symptoms similar to lead toxicity, which may explain the observations by Inuit people of strange behaviors on the part of the other crew members later in the expedition.

The HMS Erebus and HMS Terror, the two ships that embarked on Franklin's expedition, were discovered in Canada's North in 2014 and 2016, respectively.

Exploration of the Terror showed the vessel had been carefully shut down by the crew, who then boarded the Erebus and sailed south where they met their tragic fate.